docs: add .agents workspace and the Astro refactor plan

Adds the agent-facing workspace and a 20-task plan for migrating the site
to Astro. Nothing here implements the refactor; these are briefs, rules and
templates that the task agents read.

- .agents/ holds context, rules, checklists, skills, specialist agents,
  component/page/config templates and gate scripts. It is vendor-neutral so
  MiniMax, Gemini and Codex can all read it; CLAUDE.md just points at
  AGENTS.md.
- .husky/ plus .lintstagedrc.json wire the three gate tiers. gate.sh locks on
  the shared git-common-dir so parallel worktrees serialise, and guards the
  assertion count in scripts/verify.mjs against a coverage drop.
- plans/astro-refactor/ carries the phase graph, per-task briefs and the
  model-routing recommendation.

These files must be tracked before fanning out: a worktree only checks out
tracked files, so an untracked plan is invisible to every agent working in one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/templates/config/gitea-ci.yaml

@@ -0,0 +1,39 @@
+# Tier 3 gate. Copy to .gitea/workflows/verify.yml in task 01.
+#
+# Known trap: this Gitea's act-runner registration lives in an emptyDir, so a
+# pod restart silently kills CI. If the site stops updating, check the runner
+# BEFORE debugging the workflow.
+name: verify
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  gate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0   # gate.sh compares assertion counts against origin/main
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+
+      - run: npm ci --prefer-offline
+      - run: npm run lint
+      - run: ./.agents/scripts/gate.sh
+
+      # Tier 3 only: too slow for pre-push, essential before publishing.
+      - name: visual regression
+        run: |
+          npx playwright install --with-deps chromium
+          node .agents/scripts/visual-regression.mjs
+
+      - uses: actions/upload-artifact@v4
+        if: failure()
+        with:
+          name: screenshots
+          path: .agents/snapshots/diff/